1. Field of the Invention
The present invention relates to an apparatus for measuring components of liquid samples based on the ability of these components to absorb infrared radiation at certain frequencies. The apparatus according to the invention which may be used for analyzing liquid samples in general is specifically, but not exclusively, intended for use in analyzing liquid milk samples or determining components thereof.
2. Description of the Prior Art.
Analyzing apparatuses for automatic determination and registration of various components, such as fat, protein, and lactose, of milk samples being exposed to infrared radiation, are known. In such known apparatus infrared radiation from a radiation source is divided into two separate beams which by means of a system of mirrors are passed through a component filter and a reference filter, respectively. The component filter admits a narrow band of infrared wave lengths at which the radiation absorbing ability of the component to be measured is relatively high, and the reference filter admits a narrow band of wave lengths at which the radiation absorbing ability of said component is substantially lower. These filtered infrared radiation beams are interrupted by a rotating reflecting chopper disc at a frequency of 12.5 Hz, and the component and reference light pulses thus generated are directed through a transparent cuvette containing the milk sample and thereafter focused on a pyroelectric radiation detector which generates an electric signal in response to the radiation pulses received. A radiation attenuating comb-like member extends into the reference beam and may be displaced by a DC servomotor which is energized by the electrical signal from the radiation detector. When unbalance of the sample and reference beams occurs because of a change in sample component absorption the amplified signal causes the servomotor to drive the attenuating comb-member further into or out of the reference beam until balance is restored. The component of the sample is then determined as a function of the position of the comb-like member in the reference beam when balance has been obtained.
The known analyzing apparatus with the dual beam system described above is of a rather complicated structure containing ten different mirrors the positions of which must be accurately adjusted. Furthermore, the accuracy of a dual beam system implies i.a. that the condition of the beam directing mirrors associated with each of the two beams does not change non-uniformly, for example due to dust or other changes in reflection ability.
The prior art also comprises an infrared single beam analyzer for determining one component of a gas flowing through a transparent gas cell. An infrared radiation beam is directed from an infrared source through the gas cell by means of optical lenses and focused on an indium antimonide detector. The radiation beam is chopped at a chopping frequency of 600 Hz, and interference filters selected for the measurement and reference wave lengths are interposed alternately in the radiation beam at a frequency at about 6 Hz so that the detector receives chopped energy at a level corresponding alternately to the measurement and reference transmission levels. Consequently, the output signal of the detector is a 600 Hz carrier modulated at 6 Hz. The signal generated by the detector is supplied to signal processing circuits for determining the component to be measured on the basis of said signal.